Use Qt with OpenGL on Windows (#229).

Indeed, this is needed for Windows XP (see
http://qt-project.org/wiki/Qt-5-on-Windows-ANGLE-and-OpenGL).
Some work on Qwt's MathML renderer.
Some work on Qwt's MathML renderer.
Qt 5.1.1 fixes an issue that got introduced in Qt 5.1.0 (closes #).
Some work on Qwt's MathML renderer.
Some work on Qwt's MathML renderer.
Minor update to the developer documentation.
Some minor cleaning up.